It is well known in the field of electrosurgery that there are two distinct tissue effects which can be achieved by RF, depending on the temperature to which the tissue is raised. The first is the removal of tissue by vaporisation, in which the electrode or electrodes are subjected to relatively high temperatures (typically over 1000° C.). The second is the production of tissue necrosis without the removal of tissue in situ, and it is to this second type of system to which the present invention is directed.
U.S. Pat. No. 5,843,021 discloses a typical example of this type of treatment, in which a probe including a tissue treatment electrode is placed in contact with body tissue, and an RF signal is supplied to the probe such that the electrode heats the tissue causing cell necrosis and forming a lesion. The lesion is subsequently absorbed by the body with the result that tissue shrinkage is seen to occur. This type of treatment has been successfully performed for a number of years, and is known as “Somnoplasty”.
It is well known that to produce a lesion the temperature of the tissue should be kept below 100° C. Temperatures above 100° C. are known to cause charring and desiccation of the tissue (which can be undesirable as the desiccated tissue is unable to absorb any further RF energy). The prior art teaches that temperatures in the range of 80° C. to 100° C. are typical for this type of apparatus. Examples of prior art patents teaching temperatures in this range are U.S. Pat. No. 6,126,657, U.S. Pat. No. 4,411,266, U.S. Pat. No. 5,549,644, U.S. Pat. No. 5,456,682 and U.S. Pat. No. 6,056,745. As can be seen from these and any many other prior art documents, the instruction to maintain the temperature below 100° C. is well established. For example in U.S. Pat. No. 4,411,266 it is stated “any non-uniform hot spots must be monitored to prevent runaway flash heating to the boiling point of 100° C.” U.S. Pat. No. 6,056,745 states “The maximum temperature condition T.sub.MAX lies within a range of temperatures which are high enough to provide deep and wide lesions (typically between about 90° C. and 98° C.), but which are safely below about 100° C., at which tissue desiccation or tissue boiling is known to occur.” As can be seen, the direction to maintain the probe temperature below 100° C. is seen as an essential requirement for successful lesion generation.
The problem with this requirement to maintain the probe temperature below 100° C. is that this can lead to a relatively slow process, requiring anything up to several minutes of treatment time in order to generate the lesion. Unless the control of the energy delivered to the probe is exceptional, the safest way to ensure that the probe temperature does not under any circumstances exceed 100° C. is to drive the device such that its normal operating is well below 100° C. Many devices operate at temperatures of between 80° C. and 85° C. (see U.S. Pat. No. 4,411,266, for example), which can lead to even longer treatment times. The present invention seeks to provide a lesion generation system which can produce effective lesions using considerably reduced treatment times.